This invention relates to improvements in a hot wire metal electrode inert gas (hereinafter MIG), welding torch, and to an improved method of consumable electrode arc welding for carbon steels, alloy steels, aluminum and its alloys, and other types of metallic materials. The invention is applicable to semi-automatic welding practice.
Many attempts to improve consumable electrode arc welding techniques have been made with the object of attaining high speed and efficiency, yet maintaining the high quality of conventional welding methods.
For instance, a method in which a plurality of electrodes are mounted inside a shield nozzle and a welding operation is carried out while the phase of a pulsed voltage or pulsed current supplied to each of said electrodes is shifted, has been proposed in a Japanese laid-open patent application (Kokai) No. 63-313674. Another type of welding equipment was described in Kokai No. 59-16680, in which the molten consumable electrode forms a spray pattern. This device has a means for supplying a separate metal filler wire into an arcuate area in such a way that the filler wire can contact the molten portion of the base metal being welded. The filler wire is heated by resistance, and there is a control means for adjusting the heating current in the filler wire to less than one-half of the welding current.
Although the aforementioned methods employ a pulsed voltage or a pulsed current in order to prevent arcing interference between the electrodes and to avoid interference between the consumable electrode and the filler wire, the control apparatus is complicated. As an additional example, another application, Kokai No. 63-20184, proposed apparatus in which the electrodes of two torches are positioned very close to one another; on one side the electrodes are in contact with the anode of the welding direct current supply, and on the other side the electrodes and base metal were connected to the cathode. In Kokai No. 63-313674 a plurality of electrically insulated electrodes is installed within a shield nozzle and the pulsed voltage, as well as the pulsed current, is phase-shifted and distributed such that each electrode in turn has a peak period, in turn, during a welding operation.
Hot wire MIG practice, described in the aforementioned Kokai Nos. 59-16680 and 63-20184 is superior to conventional MIG welding in that it allows an increased welding rate. However, hot wire MIG welding requires two welding torches of relatively large dimensions; one for the consumable electrode and the other for a filler wire.
While the prior art methods are applicable to a fully automatic welding machine, they are not suitable for semi-automatic welding in which a welder is required to carry or hold the welding torches during the procedure. High-speed, semi-automatic welding has long been desired in the art, but has not yet been achieved.
Another problem in the art which hinders high-speed welding operations is undercutting. When severe, undercutting will cause the formation of irregular beads in the weld which results in inconsistent weld quality. This phenomenon is believed to be caused by plasma current generated by the arc which gouges the molten base metal and affects the wettable interface at the walls of the molten metal bath developed during the arc welding process.
In high-speed welding it is generally necessary to increase the welding current in order to achieve a suitable penetration depth and metal deposit quantity. On the other hand, it is recognized that increasing the welding current results in increased plasma current with attendant risk of undercutting and formation of irregular beads. Control of the welding current has proven to be difficult. While high-speed welding can be achieved by employing the multi-electrode welding method described in Kokai No. 63-313674, formation of stable welding beads remains difficult because of the aforementioned problems of bead formation and also excess sputtering.
It is important to recognize that increasing the speed of deposition is not equivalent to increasing the rate of welding. Thus, even if deposition speed is accelerated, high-speed welding cannot be attained when welding current is increased to obtain sufficient penetration. Thus, increasing the speed of deposition confers higher welding efficiency, but not necessarily an increase in welding rate.
When welding aluminum and its alloys, additional weld defects are known which include cracking, porosity, weld distortion, and puckering. There are a number of known useful techniques to minimize these problems. These include reducing the weld heat input as much as possible, controlling the bath temperature, and applying reverse distortion. However, utilizing such techniques reduces welding performance.
The puckering phenomenon, which occurs when welding aluminum and its alloys, is described, for example, in the "Journal of Light Metal Welding", Volume 22, No. 9, pp. 395-407 (1984). This generally takes place under excessive weld current or with insufficient shielding. To prevent puckering of aluminum welds, controlling weld current or increasing the amount of shield gas are considered to be effective countermeasures. Again, these countermeasures cause a reduction in welding efficiency and increase the required amount of shield gas, resulting in uneconomical welding practice.
In a conventional semiautomatic welding torch, as illustrated in FIG. 9, a consumable electrode wire 23, which is supplied by a wire feeding device (not shown) through a flexible conduit 22, is passed inside a conductive contact tube 24, disposed in a welding torch 21. Shield gas is supplied through a gas hose 25 and is directed to a weld through cylindrical gas nozzles 26, which are coaxially mounted around the conductive contact tube 24. A welding current is conducted to a contact point on the consumable electrode wire 23 through a lead wire 28 inserted within a cooling water hose 27 through conductive contact tube 24. This current arcs across a gap to the base metal (not shown).
Cooling water is supplied through cooling hose 29 in order to cool nozzle 30. The cooling water is then exhausted into cooling hose 27 to cool the lead wire 28.
In the welding torch described above, only consumable electrode wire is generally supplied. Therefore, in order to perform hot wire MIG welding, separate torches for consumable electrode wire and filler are required.